The main suspect in the 1996 bombing of the Khobar Towers in Saudi Arabia is reported to have been captured.
Ahmed al-Mughassil was apprehended in the Lebanese capital Beirut and then transferred to Riyadh, the Saudi-owned newspaper al-Sharq al-Awsat reported.
There was no immediate comment from the Saudi authorities, but a US official confirmed Mr Mughassil was being held.
He has been indicted by a US court for the attack, which killed 19 US military personnel and a Saudi citizen.
The US has accused Iran of being behind the bombing - something it has denied.
In a report published on Wednesday, al-Sharq al-Awsat said Mr Mughassil, a Saudi national, had been "disguised in a way that made it hard to identify him".
The newspaper quoted official Saudi sources as saying Saudi security personnel had received information about the 48-year-old's presence in Beirut. It did not give further details about when or by whom he was detained.
Later, a US official told the Associated Press that Mr Mughassil was in custody.
The US had offered a $5m (£3.2m) reward for information leading directly to the apprehension or conviction of Mr Mughassil, who it alleges was the head of the military wing of Saudi Hezbollah, also known as Hezbollah al-Hijaz, a branch of the Iran-backed Lebanese Shia Islamist movement that operated in the Sunni-ruled Gulf kingdom.
Mr Mughassil was one of 13 people named in a 2001 indictment filed at a US District Court in Virginia.
It says that on 25 June 1996 Hezbollah al-Hijaz members drove a tanker filled with plastic explosives into the car park at the Khobar Towers housing complex near the eastern city of Dhahran and detonated it, all but destroying the nearest building.
In 2006, a US District Court judge ordered the Iranian government to pay $254m in damages to the families of 17 of the 19 Americans killed in the attack after it failed to respond to a lawsuit.
The judge said he had been persuaded by former FBI agents that the truck bomb had been assembled at a base in Lebanon's Bekaa Valley operated by the militant Shia Islamist movement, Hezbollah, and Iran's Revolutionary Guards. He also concluded that the attack had been approved by Iran's Supreme Leader, Ayatollah Ali Khamenei.
However, the 9/11 Commission Report said in 2004 that "while the evidence of Iranian involvement is strong, there are also signs that al-Qaeda played some role, as yet unknown".